VDB - Shadow bug and emission question

[TomG]

I'll ask the devs and find out one way or the other :)

[martinsik]

As for the missing shadow in the first image of this thread, this is an old Corona issue, which is related to all volumes (not only OpenVDB). This will be difficult to fix, unfortunately. For now, just make sure that objects don't intersect the volume grid bounding box (they either lie completely outside or completely inside the volume).

As for the PhoenixFD temperature, it is for some reason stored as very high numbers compared to temperature exported from other software (FumeFx or Blender etc..)
The only thing I can do about this is set the input scaling automatically when I somehow detect that the OpenVDB file was created by PhoenixFD.

[cecofuli]

Hi martinsik,
thank you for the explanation. I think that you should find a solution for a better compatibility with PhoenixFD.
Not it's very difficult to create some good Emission shader with the Temperature channel.
Maybe you can ask help to Chaosgroup

Have you tried my .vdb sequence? Even with very low Scale value, it's impossible to obtain what I had directly with PhoenixFD.

[cecofuli]

Speaking with Martin:

"We will be redoing the UI of CoronaVolumeGrid and we will also address the issue with the temperature".

Great support, as usual. ^_^

[TomG]

Looks like the Emission from Phoenix FD should be fixed in the latest daily (it was related to how Phoenix does black body radiation by the look of the comment in the changelog):

"emission data can now be optionally interpreted as temperature (calculating black body radiation color) + associated temperature scaling - this is automatically detected for Phoenix FD files and for others containing "temperature" channel"

[cecofuli]

Ok, I tried again with the last Corona Daily and PhoenixFD v3.10 (no daily)
It's very, very uncomfortable to work with PhoenixFD file.
I have to use very too low value (Scale= 0.02 and Input scale 0.001).
When I try to change Input scale to 0.002... BOOM... Nuclear explosion =) !!!!
Also to play Gradient it's very annoying...  I have to play with only the 15px, because 80% of the space is a gradient between black and red...
And look at the Curve graph... so flat for70% of the space.
I will wait to play with the last official release of PhoenixFD (^__^)

[martinsik]

PhoenixFD exports emission as temperature in Kelvins, so it recommended to set emission Mode to Blackbody, which automatically computes color from the temperature.

[cecofuli]

Ah, ok... but, with Blackbody I cannot "play" with any Channel color mapping... =)
EDIT: we can use the Emission Custom map, like a simple 3ds Max Gradient =)

[maru]

Some quick notes:
-The examples at the top basically show fire/explosion during daylight situation. Have you seen fire (e.g. campfire) during daylight? The fire is often invisible. :) So I would suggest switching all lights off, and experimenting with fire/explosions in the darkness, or with a single, dim light source
-Emission - like Martin said, setting its mode to blackbody helps a lot, and makes it more predictable

[cecofuli]

Mara, and, if I want to reproduce something like this? =)



Yes, it's more predictable, but we lose the possibility to use Channel color mapping.
To use Gradient Ramp isn't very complex, but it was better to play with the Channel color mapping ;-)

[TomG]

The new Blackbody option in the CoronaVolumeGrid should fix this - you need to swap from the old Channel mapping mode to Blackbody (see screengrab, this scene used to require the super small values in the Channel Grid etc) and then use Temperature scale to manage the brightness of the Emission.

[TomG]

Would be interested to see your test scene with that - though I note you do say you know of it, just that you miss the Channel Grid mapping (what would you use it for, though - curious as to why it might still be useful in these situations?)

[cecofuli]

Hi Tom,
yes, I know, and Blackbody works nicely.

As for your question: for example to crate "magic" explosion/smoke  ;-)



You can use, with Blackbody, a custom Gradient Ramp.
But I don't understand why we cannot use the nice built-it Channel color mapping when we are using Blackbody option =)
But... it's ok =)

[TomG]

TY for the intended use! Magic fire and smoke is nice :)

[maru]

I experimented with the volume grid a bit more and it seems that the channel color mapping is pretty predictable when adjusting absorption and scattering, but indeed hard to understand and tune when adjusting emission of objects exported from PhoenixFD.
It is much easier to grasp when adjusting emission of objects downloaded from OpenVDB. So (as it was already explained here) it seems that the scale (intensity?) of the temperature matters here and the solution to deal with PhoenixFD objects is to use the blackbody mode.

I think Francesco's argument is valid - it would be good to have some user friendly controls to easily adjust the gradient and intensity of the emission regardless of the temperature intensity. I do not understand why it's not working so, even though it is supposed to map the original temperature range to 0-1 range. Maybe controlling the color gradient for the blackbody mode would be a good workaround?